In a hitherto used freezer unit of the so-called two dimensional refrigeration system of this kind, refrigerant circuits on high and low temperature sides are constituted of respective refrigerant close circuits of two independent lines to configure heat exchangers by an evaporator in the refrigerant circuit on a high temperature side and a condenser in the refrigerant circuit on a low temperature side so that the refrigerants in the refrigerant circuit on the low temperature side are condensed by evaporation of the refrigerants in the refrigerant circuit on the high temperature side (refer to, for example, Japanese Utility Model Application Laid-Open No. 23,101/1983). In this way, as it becomes possible to use refrigerants of lower boiling points (evaporating temperatures) in the refrigerant circuit on the low temperature side, extremely low temperatures can be obtained by means of the evaporator in the refrigerant circuit on the low temperature side.
According to such a two-dimensional refrigeration system, low temperatures of the order of −80° C. are usually achieved in the evaporator in the refrigerant circuit on the low temperature side. In order to obtain even lower temperature, for example, such as −150° C., it would be required to improve the refrigerant circuits or take various measures for compositions of the refrigerants to be confined in the circuits.
The applicant of the present application had realized extremely low temperatures such as −150° C. by improving the compositions of the confined refrigerants as disclosed in an official gazette of Japanese Patent No. 3,208,151. In more detail, the low temperatures had been realized by confining mixed refrigerants including R22 (chlorodifluoromethane: CHClF2), R142b (1-chloro-1,1-difluoroethane: CH3CClF2) and R21 (dichlorofluoromethane: CHCl2F) into a refrigerant circuit on a high temperature side and mixed refrigerants including R21, R22 and R23 (tolyfluoromethane: CHF3), R14 (carbone tetrafluoride: CF4), R50 (methane: CH4) and R740 (argon: Ar) into a refrigerant circuit on a low temperature side.
According to the compositions described above, however, the refrigerants such as R21 and R22 belonging to a so-called HCFC contain chlorine atoms which are presumed to be source of destruction of the ozone layer so that they are subjects to the ozone regulation. Consequently, there is a need for development of compositions for effective replacement of the hitherto used refrigerants, which can maintain performance of refrigerant circuits without modifying the conventional circuits, and without the risk of destruction of the ozone layer.
Incidentally, the R21 hitherto used is highly compatible with oil (alkyl benzene), and therefore, it serves to return the oil to a compressor as an oil carrier, thereby preventing defective lubrication or locking of the compressor. As the R21 is a refrigerant containing chlorine as described above, however, any other refrigerant is required substituting for the R21.